The present invention generally relates to radio frequency transmitters, and particularly relates to improving performance in transmitters employing direct modulation of amplitude and phase.
Communication systems represent a complex balance of competing interests. Today's modern digital wireless communication systems stand as dramatic testimony to the increasing complexity and intricate blending of solutions required to field reliable, high performance voice and data communication systems. Engineers must balance requirements of bandwidth and power efficiency against signal fidelity and linearity requirements, all while meeting increasingly strict power and interference requirements.
For example, the need to squeeze a greater number of simultaneous users into a limited allocation of radio spectrum bandwidth creates complications regarding signal modulation and transmission. With closely spaced radio channels and high linearity requirements, cross-channel interference looms large. This potential for interference requires system designers to adopt signal transmission schemes that substantially limit spectral spreading or splatter between the different radio channels.
Simultaneously, users of these communication systems expect small, convenient communication devices that operate for long hours before requiring a battery recharge. In the quest to offer longer battery life, engineers eke out the maximum efficiency from each of the sub-systems within these communication devices. Almost nothing escapes the attention of these engineers as they strive to reduce device power consumption, from the power conversion and regulation circuits, to the audio input and output circuits. Because a preponderance of power typically goes to the radio frequency (RF) transmission portions of these communication devices, particularly to the RF power amplifier (PA) sections, engineers understandably devote much effort to increasing RF transmission efficiency.
In general, a RF power amplifier operates with varying efficiency across its range of operating modes, where efficiency is expressed as the ratio of output RF power to input power. Linear mode operation of the power amplifier exhibits lower efficiency than saturated mode operation but with the obvious benefit of linear signal amplification. Transmit signal linearity plays an important role in preserving signal fidelity and limiting cross-channel interference. Indeed, essentially all modern digital wireless communication standards, such as the TIA/EIA/IS-136, TIA/EIA/IS-95B, TIA/EIA/IS-2000, EDGE, W-CDMA air interface standards, require linear signal amplification.
Direct modulation of amplitude and phase (DMAP) techniques allow, in at least some configurations, a greater RF transmitter efficiency while preserving modulation linearity. Most digital modulation standards include both phase modulation information and amplitude modulation information. That is, the transmitted signal conveys desired transmit data using both phase (or frequency) and amplitude modulations. With DMAP, a processing system separates the phase modulation information from the amplitude modulation information. Generally, the phase modulation information comprises a constant-envelope signal used to phase modulate a RF carrier signal having the desired transmit signal frequency. This phase-modulated carrier signal is then envelope modulated using the corresponding amplitude modulation information.